Abstract
Since the well-known symmetry has its fundamental significance and implication in physics, where denotes a joint operation of space inversion and time reversal , it is important and intriguing to explore exotic -invariant topological metals and to physically realize them. Here we develop a theory for a different type of topological metals that are described by a two-band model of -invariant topological nodal loop states in a three-dimensional Brillouin zone, with the topological stability being revealed through the -symmetry-protected nontrivial topological charge even in the absence of both and symmetries. Moreover, the gapless boundary modes are demonstrated to originate from the nontrivial topological charge of the bulk nodal loop. Based on these exact results, we propose an experimental scheme to realize and to detect tunable -invariant topological nodal loop states with ultracold atoms in an optical lattice, in which atoms with two hyperfine spin states are loaded in a spin-dependent three-dimensional optical lattice and two pairs of Raman lasers are used to create out-of-plane spin-flip hopping with site-dependent phase. It is shown that such a realistic cold-atom setup can yield topological nodal loop states, having a tunable band-touching ring with the twofold degeneracy in the bulk spectrum and nontrivial surface states. The nodal loop states are actually protected by the combined symmetry and are characterized by a -type invariant (or topological charge), i.e., a quantized Berry phase. Remarkably, we demonstrate with numerical simulations that (i) the characteristic nodal ring can be detected by measuring the atomic transfer fractions in a Bloch-Zener oscillation; (ii) the topological invariant may be measured based on the time-of-flight imaging; and (iii) the surface states may be probed through Bragg spectroscopy. The present proposal for realizing topological nodal loop states in cold-atom systems may provide a unique experimental platform for exploring exotic -invariant topological physics.
- Received 6 January 2016
DOI:https://doi.org/10.1103/PhysRevA.93.043617
©2016 American Physical Society